1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof.
2. Description of the Related Art
In recent years, a semiconductor device has been actively manufactured, in which thin film transistors (TFTs) are formed over a substrate having an insulating surface, such as a glass substrate, and the thin film transistors are used as switching elements or the like. The thin film transistors are formed so that island-like semiconductor films are formed over a substrate having an insulating surface by a CVD method, a photolithography step, or the like, and parts of the island-like semiconductor films are used as channel forming regions of transistors. (for example, Patent Document 1: Japanese Published Patent Application No. H08-018055)
FIGS. 17A to 17C are schematic views of a general thin film transistor. First, a thin film transistor has, over a substrate 901, an island-like semiconductor film 903 with an insulating film 902 serving as a base film interposed therebetween, and a conductive film 905 serving as a gate electrode with a gate insulating film 904 interposed therebetween. The conductive film 905 is provided so as to cross the island-like semiconductor film 903. The semiconductor film 903 has a channel forming region 903a formed in a region which overlaps with the conductive film 905, and an impurity region 903b forming a source region or drain region. Further, a conductive film 907 forming a source electrode or drain electrode is provided so as to be electrically connected to the impurity region 903b. FIGS. 17B and 17C show cross-sectional structures taken along lines C1-D1 and C2-D2, respectively, in FIG. 17A.